CODEWITHSUNDEEP
pythoncalculatortrigonometry
xMutiiNy
xMutiiNy

Reputation: 9

Python 3.3 Side Cosine Rule Calculator Error

I'm trying to make a calculator in Python but I'm getting and error with the Cosine Rule I have put in.

This is what I have

x = float(input("First Side "))
y = float(input("Second Side "))
z = float(input("Angle which isn't opposite First or Second Side "))
print (" ")
print ("Side is: "+str(math.sqrt(((x**2)+(y**2))-(2*x*y*(math.cos(z)*(180/math.pi))))))

And this is my error

Traceback (most recent call last):
  File "D:/Users/---------/Python/test calc.py", line 339, in <module>
    print ("Side is: "+str(math.sqrt(((x**2)+(y**2))-(2*x*y*(math.cos(z)*(180/math.pi))))))
ValueError: math domain error

You work out the Cosine Rule by doing this:

a=√(b^2+c^2−2*b*c*cos(α))

In what I've done

x=b
y=c
z=α

Upvotes: 1

Views: 454

Answers (3)

Yosh
Yosh

Reputation: 2742

It seems math.cos(z)*(180/math.pi) is the part mistaken, making what's inside the math.sqrt negative for some cases.

print ("Side is: "+str(math.sqrt(((x**2)+(y**2))-(2*x*y*math.cos(z)))))

or math.cos(z*math.pi/180) will be what you want.

(EDIT: Corrected the upside-down fraction. Thanks: andrew cooke))

Upvotes: 1

andrew cooke
andrew cooke

Reputation: 46872

in the line

2*x*y*(math.cos(z)*(180/math.pi))

you seem to be trying to convert from degrees to radians? but (1) the factor is upside down and (2) it needs to be inside the cos(...).

so you may have meant

2*x*y*math.cos(z*math.pi/180)

(check: when z is 180 degrees, the code above gives pi radians).

Upvotes: 2

thumbtackthief
thumbtackthief

Reputation: 6221

It works for me; I'd imagine that you are putting in numbers that are impossible for a triangle. Try your input in a regular calculator and see if it works.

Also, note that your angle needs to be in radians (or you need to convert it to degrees).

I tried sides of 3, 4 and 3.14/2 (approximately 90 degrees in radians) and got 4.99, which approximates the correct answer of 5.

Upvotes: 2

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